This invention relates in general to apparatus and processes for agglomerating carbon dioxide snow particles and accelerating the snow particles against a surface to be cleaned.
Various techniques have been devised to cleaning debris such as submicron particles from substrate surfaces. The semiconductor industry has employed high pressure liquids alone or in combination with fine bristled brushes to remove finely particulate contaminants from semiconductor wafers. These processes have achieved some success in removing contaminants, but the brushes scratch the substrate surface and the high pressure liquids tend to erode the delicate surfaces and can even generate an undesirable electric discharge. Also the liquids can not readily be collected after use in brush and high pressure liquid systems.
An improved cleaning system has been discovered in which a mixture of substantially pure solid and gaseous carbon dioxide removes submicron particles from substrate surfaces without the disadvantages associated with the above-described brush and high pressure liquid systems. Pure carbon dioxide (99.99+%) is expanded from the liquid state to produce dry ice snow which can is blown across a surface to remove submicron particles without scratching the substrate surface. The carbon dioxide snow vaporizes when exposed to ambient temperatures leaving no residue and thereby eliminating the problem of fluid collection.
More recently, apparatus for making carbon dioxide snow and for directing a solid/gas mixture of carbon dioxide to a substrate is described in Hoenig, Stuart A., "Cleaning Surfaces with Dry Ice" (Compressed Air Magazine, August, 1986, pp 22-25). By means of this device, liquid carbon dioxide is depressurized through a long, cylindrical tube of uniform diameter to produce a solid/gas carbon dioxide mixture which is then directed to the substrate surface. A concentrically positioned tube is used to add a flow of dry nitrogen gas to thereby prevent the build-up of condensation.
In U.S. Pat. No. 4,806,171 to Whitlock et al, an orifice is used to provide a pathway for the flow of fluid carbon dioxide into a coalescing chamber where the fine liquid droplets first form and then coalesce into large liquid droplets which are the precursor of the minute solid particles of carbon dioxide which are not normally resolvable by the human eye. The large droplets are formed into solid particles as the feed passes from the coalescing chamber through a second orifice and out of the exit port toward a substrate surface.